Smelting copper in decorated pottery: communities of practice in the Niari Basin, Republic of the Congo, fifteenth–seventeenth centuries CE

This paper considers copper production in the Niari Basin, Republic of the Congo, during a period dated to the mid-fifteenth–mid-seventeenth centuries CE. Using a combination of pXRF, OM, SEM–EDS, and FTIR, it assesses the microstructure and composition of slags and technical ceramics from sites associated with two different regional pottery traditions: Moubiri-type at the site of Kingoyi near Mindouli and Kindangakanzi-type at Kindangakanzi near Boko-Songho. Both sites are characterised by the use of refractory domestic pottery as crucibles for copper smelting. Moubiri-type pottery is alumina-rich, while Kindangakanzi-type pottery is formed from a magnesia-rich clay, a crucible type unique in sub-Saharan Africa. Similarities in chaînes opératoires at Kingoyi and Kindangakanzi suggest sharing of knowledge at mining and smelting sites, interactions we reconstruct as a metallurgical constellation of practice comprised of the distinct potting communities of practice (see Supplementary information for abstract in Lingala and French). Supplementary Information The online version contains supplementary material available at 10.1007/s12520-022-01653-9.


Site descriptions
The sites of the 15 th -17th century, as elsewhere in the Niari, suffer from extreme erosion. Kingoyi lies to the southwest of Mindouli, among a group of sites on the Plateau des Cataractes (Fig. 1). Over 7.75 kg of metallurgical debris was recovered at the site, including an aggregate of slag/soil (Nikis, 2018, vol. 2, table 3.2, p. 54). A fragment of sandstone was perhaps used for crushing ores and/or slags. Radiocarbon dating of charcoal from layer 3, which included the aggregate, yielded a 2σ date of 1465-1636 cal AD.
Kindangakanzi is located among a series of hills northeast of Boko-Songho. The site was poorly stratified but contained an ellipsoidal bowl furnace (Fig. 2). Over 2.75 kg of metallurgical debris (slags, tuyères, crucibles, and ores) was recovered (Nikis, 2018, vol. 2, table 17.2, p. 251). Radiocarbon analysis of charcoal from the furnace established a 2σ date of 1457-1627 cal AD. Lead isotope analysis of slag from Kindangakanzi indicated that it is consistent with the lead-rich copper ores from Djenguelé, a mine ca. 2 km from the site (Rademakers et al., 2018).
The crucible assemblages of both sites are highly fragmentary, with sherds only a few cm in size. Around 10-20% of ceramics at Kingoyi are slagged, and 30-60% from the various sondages at Kindangakanzi are slagged (Nikis, 2018, vol. 2, pp. 53, 248). Tuyères are less abundant at Kingoyi than at other sites near Mindouli (15% of metallurgical debris by weight, cf. 75% at Ntominsié); tuyères around Mindouli are similar in dimension, suggesting standardisation (Nikis, 2018, p. 308). Tuyères are common at Kindangakanzi (ca. 60% by weight).  Screening pXRF analysis were performed on unprepared surfaces using Olympus Vanta VMR instruments. Two different instruments of the same model and comparable settings were used. For each sample, between 1-3 analyses were conducted per surface (slag, ceramic). Analyses were performed using the 'GeoChem' mode, a fundamental parameters calibration supplemented with empirical optimisation and including two beams: one at 40 kV with an Al filter, and and another one at 10 kV with no filter for lighter elements, with measurement times of 30 s per beam. Beam currents are automatically adjusted by the system for each analysis, ranging 63-78 µA ( We analysed a soil standard using one of the instruments, demonstrating that light elements tend to be underestimated but that overall accuracy is acceptable for the elements used in our discussion (Table 4). Unfortunately, the same standard was not available when the second instrument was employed, but the use of identical settings and the correspondence between groups irrespective of the analytical batch is indicative of consistency in the results. It should be noted, however, that we analysed unprepared surfaces in order to preserve artefact integrity, and therefore analytical uncertainty and error are likely higher than suggested by the analysis of standards. In any case, SEM-EDS of a subset of samples prepared as polished blocks confirmed the chemical patterns and group attributions based on pXRF, which reinforces the validity of our sequential approach to sampling and analysis.

Comparison of pXRF and SEM-EDS results
Exact correspondence between pXRF and SEM-EDS results cannot be expected, given the different sampling areas, but comparison of both datasets shows broad consistency. (Fig. 3).
With the exception of Al2O3 in metallurgical slag, in general pXRF values are slightly lower, probably because of the higher porosity and irregularity of the surfaces leading to lower analytical totals.

Ore geology
Three samples of ore were analysed, two from Kingoyi and one from Malembe around Boko-Songho (see SI Catalogue). The fragments are <4 cm in maximum dimension. GPSNN272-5 from Malembe contains malachite intergrown with talc and calcite/dolomite. The ores from Kingoyi are not malachite. MKU3b14_7 is principally dioptase. MKU3b14_9 is principally goethite, found in gossans and thus typical of weathered copper mineralisations, interlaced with veins of dioptase (Fig. 4). This composition is reflected in high FeO in the SEM-EDS bulk data presented in the main text ( Table 5). The presence of iron-rich ores at Kingoyi is noteworthy, as iron concentrations in metallurgical debris are low.
Previously published chemical data indicated minor amounts of manganese (<2000 ppm) in Niari ores, elevated concentrations of silver (<3170 ppm) and arsenic (<6100 ppm) around Mindouli, in particular, and high lead values (<33%), including complex malachite-galena ores, around Boko-Songho (Rademakers et al., 2018). The ores examined here contain manganese, but are broadly similar in terms of silver, arsenic, and lead, likely due to the limitations of the pXRF screening approach.

Pottery
The difference between the kaolinitic Moubiri-type pottery and the smectitic/saponitic Kindangakanzi-type is evident in the FTIR spectra of MKU3b15_13 (ceramic) and KNA14_8 (see SI Catalogue).
Some Kindangakanzi-type samples (KNA14_9, KNA14_29) display angular talc/saponite down to < 10 μm in KNA14_9 and KNA14_29 suggesting a residual clay, i.e., formed from the weathering of a talc vein (Quinn, 2013, pp. 119-122). On the other hand, other samples (KNA14_8, KNA14_11) display rounded inclusions more typical of secondary clays, i.e., from within a river valley. KNA14_8 contains both distinct talc oolites and a fragment of siltstone, a mixed signature from the two main geological formations: oolitic dolomite (Bangu) and siltstone (Mpioka) (see SI Catalogue). MKU3b14_3, a Kindangakanzi-type crucible found at Kingoyi, is akin in terms of macroscopic appearance, refractoriness, and trace element composition, but also contains apatite (Fig. 5).
There are peaks in the FTIR spectra of unslagged sherd KNA14_8 (see SI Catalogue) that appear as talc alters to enstatite as well as the development of a shoulder on the Si-O-Si peak, both of which occur around 800° C (Berna et al., 2007;Liu et al., 2014;Weiner, 2010, p. 305). This estimate is merely indicative that the vessels were not subject to a high pre-firing before their reuse as crucibles.
A comparison of bulk and matrix ceramic composition indicates overall similarity, reflective of the use of unmodified clay or the addition of aplastic inclusions from the same geological formation (Fig. 6). Capacity estimates for a typical Moubiri-type and Kindangakanzi-type vessel are shown in Fig. 7.   Fig. 7 Capacity estimate using the ULB online calculator (Engels et al., 2009). Left: Incomplete profile of Kindangakanzi-type vessel (Nikis, 2018, fig. 5.21). Right: restored Type 1 Moubiri-type (Nikis, 2018, fig. 5.18). The exact number is less important, especially since the vessels were not necessarily always filled all the way to the brim, but note that even the partial volume of a Kindangakanzi-type vessel is substantially larger than a typical Moubiri one, allowing for a larger charge when used as a crucible

Tuyères
The slag layer on Kingoyi tuyère MKU3b15_2 varies between 1.4-7.0 mm thick and is homogeneous with a smooth, regular surface. The tuyère is clearly linked to copper metallurgy and contains comparable concentrations of copper and iron to other debris. The slag layer is not particularly enriched in silica or iron relative to the ceramic, indicating that these elements in the slag come from the melting of the quartz-rich ceramic, as visible in section. Prills (<1.2 mm) within the tuyère slag are considerably scarcer than within other slags, and there is trace silver present.
The Kindangakanzi tuyère KNA14_10 is heavily slagged, with a thick, dense black layer on the exterior between 1.2 and 5.5 mm thick. This slag layer contains the highest concentration of iron of residues from the site (22.0% FeO), and the slag layer is enriched in iron relative to the ceramic. The slag also contains large copper prills and 20.5% CuO, confirming its link to copper metallurgy. Notably, the tuyère residue does not contain lead.